The invention relates to a bone plate for the splinting of a fracture at a bone, comprising a plurality of bone screws which fix the bone plate to the bone with their screw head and which are radially braced in the direction towards the fracture via an elastic cushion of plastic to the bone plate in order to permit a restricted displacement in compression stressing of the bone.
In contrast to pressure plates, bone plates exert no compression during their application to the fracture of a bone. During securing with bone screws the bone parts remain unchanged in the longitudinal direction with respect to the bone plate.
A bone plate with elongated holes is shown in U.S. Pat. No. 4,943,292 (FOUX). A ring disc is provided between the securing head of a suitable bone screw and the bone plate and protrudes into an elongated hole with a projection in the form of a cushion on the side towards a provided bone fracture. An elastic plastic, which is also intended to enable sliding of the screw head when the cushion is pressed together in a compression of the bone, is provided as a material for the ring disc and the cushion. A problem in this arrangement is that the bone plate, which is pressed on the bone through the bone screw, should move relative to the bone. The tightening tension of the bone screw together with the friction between the plate and the bone determines the amount of a compression force, which leads to a displacement in the axial direction. Since the adhesive friction is practically always greater than the sliding friction, the displacement will take place in a jump-like manner as will moving back through the cushions when the compression force diminishes. A further disadvantage exists in that due to the large contact surfaces, large amounts of bone are disturbed in their blood circulation and decrease in their bone density (porosis).
The object of the invention is to improve these circumstances. The invention satisfies the object in that the screw head penetrates through the bone plate completely and lies on the bone with a contact surface which projects beyond the lower side of the bone plate by at least a distance xcex51; and in that a ring body is formed at the cushion of plastic, can be applied with the cushion of plastic at the lower side of the bone plate and projects beyond the lower side by a distance xcex52.
The invention has the advantage that, in contrast to pressure plates, which aim at a direct, callus-free primary healing, a secondary bone healing with controlled callus formation is enabled. Through the intentional distribution of the pressing force, which is active in the axial direction of the bone screw, onto a contact surface of the screw head which lies in contact at the bone and onto a ring body as an intermediate member, the frictional forces which counteract a longitudinal displacement of the bone plate are so small that the resilient action of the cushion of plastic actually comes to bear. In the fracture gap, which leads to the formation of a callus, the callus forming is stimulated through the intentionally permitted micro-movements in the longitudinal direction of the bone plate and, in the final analysis, a more rapid and solid healing is achieved.
The screw head is braced together with the bone independently of the frictional forces which counteract the longitudinal displacement of the bone plate, whereas the bias force on the bone plate, which is clamped between the screw head and the ring body, can be kept low. Since the bone plate itself is spaced apart from and hardly makes contact with the bone, no additional forces arise which influence the friction in the micro-movements.
Further improvements are achieved with additional optional features of the invention. Thus it is helpful to manufacture the ring body and the cushion of plastic in a single piece and to choose a compression modulus Ec between 500 and 3,000 MPa for the elastic action. Through the choice of a bio-absorbable plastic for the cushion of plastic and the ring, a long-time action which is adapted to the healing process results. Because the bio-absorbable material slowly decomposes in the course of a plurality of weeks, the bone plate is clamped increasingly less severely in the course of time, and a continuously increasing stress is applied to the healing bone within the framework of narrow limits. Examples of such a material are polylactides. The Boehringer Company, Ingelheim, Germany, manufactures such a polylactide under the product designation Resomer R208, which decomposes in the human body through hydrolysis in approximately 30 weeks.
A further advantage lies in that large surface pressures are restricted to small, predetermined regions at the bone, with the proportion of pressure from the ring body steadily decreasing with the decomposition of the bio-absorbable material, so that the time point for the removal of the bone plate can be further delayed without great damage. The contact surface of the ring body and the screw head on the bone is more than 100% larger than the cross-section of the nominal diameter of the thread of the bone screw. It suffices to keep this contact surface smaller than eight times the cross-section of the nominal diameter of the thread in order to find within these limits (between 100% and 800%) a contact surface which is adapted to the thread of the bone screw with respect to the contact force which can be achieved.
Because the ring bodies can in each case be inserted in only one position into the bone plate and latch in this position, when they are connected to the ring body the cushions of plastic can be mounted only on the side of the fracture. For improved conformance to the rounding of tubular bones, the bone plate and the ring body can be concavely arched in the transverse direction at their lower side.